1. Technical Field
The present invention relates to a hybrid capacitor, and more particularly, to a hybrid capacitor in which a complex structure of a lithium ion capacitor and an electric double layer capacitor is implemented in a single cell to thereby improve a manufacturing efficiency thereof and increase energy density and power density characteristics thereof.
2. Description of the Related Art
A secondary battery such as a lithium ion battery, or the like, which is a representative energy storage device having high energy density, has recently become prominent and has been used as an important energy storage device for various mobile electronic devices.
Among next generation energy storage devices, a device called an ultracapacitor or a supercapacitor has been prominent as a next generation energy storage device due to rapid charging and discharging speed, high stability, and environment-friendly characteristics.
The secondary battery has an advantage in that it has high energy density; however, it has a disadvantage in that it has limited output characteristics. The supercapacitor has an advantage in that it has output characteristics significantly higher than those of the secondary battery; however, it has a disadvantage in that it has low energy density.
In the case of the supercapacitor, a lithium pre-doping technology has been proposed in order to overcome the above-mentioned disadvantage. A lithium ion capacitor (LIC) having three to four times more energy density as compared to the energy density of the electric double layer capacitor according to the related art by using the above-mentioned technology has started to be commercialized.
Here, a kind of supercapacitors will be simply described. A general supercapacitor is configured of an electrode structure, a separator, an electrolyte solution, and the like. The supercapacitor is driven based on an electrochemical reaction mechanism that carrier ions in the electrolyte solution are selectively adsorbed to the electrode by applying power to the electrode structure. As representative supercapacitors, a lithium ion capacitor (LIC), an electric double layer capacitor (EDLC), a pseudocapacitor, a hybrid capacitor, and the like are currently used.
The lithium ion capacitor is a supercapacitor that uses a cathode made of activated carbon and an anode made of graphite, and uses lithium ions as carrier ions. The electric double layer capacitor is a supercapacitor that uses an electrode made of activated carbon and uses an electric double layer charging as a reaction mechanism. The pseudocapacitor is a supercapacitor which uses a transition metal oxide or a conductive polymer as an electrode and uses a pseudo-capacitance as a reaction mechanism. The hybrid capacitor is a supercapacitor having intermediate characteristics between the electric double layer capacitor and the pseudocapacitor.
However, the energy storage devices as described above have a relatively lower capacitance than a secondary battery. This is the reason that most of the supercapacitors as described above are driven by a charging and discharging mechanism using the movement of carrier ions on the interface between the electrode and the electrolyte solution and a chemical reaction on the surface of the electrode. Therefore, in an energy storage device such as a supercapacitor, a need currently exists for developing a technology that improves a relatively low capacitance.
Meanwhile, in both of the lithium ion secondary battery (LIB) and the lithium ion capacitor (LIC) as described above, graphite, which is a carbon material, has been mainly used as a material of an anode. Particularly, in the case of the LIC, in order to increase energy density, the anode lithiated so that it has a potential of 0.1 V or less has been used. Here, as a method of lithiating the anode, several methods may be used. However, a method of immersing the anode in an ethylene carbonate (EC) based electrolyte solution containing lithium salts has been mainly used. In this case, a solid electrolyte interface (SEI) film is formed on a surface of the graphite. This SEI film passes lithium ions therethrough and is cointercalated with solvent molecules to thereby suppress a side effect that graphite layers are peeled off. Therefore, it has been known that the SEI film is a factor having an important effect on characteristics of the LIC and the LIB.
However, since the SEI film is formed at an initial stage, an initial charging and discharging efficiency and a capacitance of the graphite having a large irreversible capacitance are inevitably reduced. In addition, in a propylene carbonate (PC) based electrolyte solution having excellent low temperature characteristics, the SEI film is not formed; rather, gas is generated. Therefore, it has been known that low temperature characteristics of the LIC become poorer than those of the EDLC. Further, in a process of doping the anode with lithium, it is difficult to perform uniform doping, a long time is required, and a performance is unstable, such that there is a limitation in commercialization. Furthermore, the LIC cannot but basically have deteriorated power characteristics due to non-polarization characteristics of the anode, as compared to the EDLC.
Meanwhile, in order to overcome the disadvantages of the secondary battery, research into and development for a technology of systematically combining the EDLC with the existing lithium ion polymer battery have been conducted. As a result of the research and development, a complex battery capable of increasing an instantaneous output and having increased energy density has been proposed. However, the complex battery has a complicated structure in view of a circuit or a manufacturing process thereof and has an increased mounting space, such that it runs counter to the trend toward miniaturization of the battery.
A need for a technology capable of basically solving the above-mentioned problems gradually increases.